An electronic security system is described in U.S. Pat. Nos. 3,810,147 and 3,863,244 for detection of the unauthorized removal of items containing a resonant tag circuit. Such system employs a transmitter providing a repetitively swept range of frequencies driving an antenna which generates a swept electromagnetic field in a zone under surveillance. A resonant tag includes a circuit resonant at a frequency within the swept band and operative in response to the applied field to resonate at its characteristic frequency which is sensed by the receiver portion of the system and processed to provide an output alarm indication of tag presence in the surveillance zone. The receiver includes signal discrimination circuits for distinguishing between an actual tag and spurious signals which could be falsely detected as a tag and therefore cause a false alarm. Preferred signal processing techniques for such electronic security systems are shown in the above-cited patents.
In an electronic security system such as that described in the aforesaid patents, any resonant circuit of a frequency within the swept band and of appropriate quality factor (Q) in close proximity to the system will actuate the alarm, since such resonant circuit is not distinguishable from and will be falsely detected as a resonant tag. In use of an electronic security system installed in a retail store, a library or other facility, the surrounding electrical environment can itself be a resonant circuit. In a particular installation, power cables, lights, copying machines and the like, can be self-resonant in the same frequency range as the swept range of the security system, and if the spurious resonance is of high Q, the system will detect this resonance and provide a false alarm. If the spurious resonance is of low Q, the sensitivity of the security system in the frequency range of this resonance will be decreased since the low Q signal will tend to distort the shape of the actual tag signal that the system is designed to detect.